1. Field of the Invention
The present invention relates to an exhaust gas purification apparatus that uses a reducing agent to remove by reduction nitrogen oxide (NOx) discharged from an engine such as a diesel engine, a gasoline engine, or the like, mounted on a mobile vehicle. In particular, it relates to an engine exhaust gas purification apparatus that prevents clogging of an injection nozzle for supplying by injection a reducing agent to an exhaust gas flow at an upstream side of a reducing catalyst, to improve the efficiency of NOx purification processing.
2. Description of the Related Art
As a system which purifies exhaust gas by removing particularly NOx from among particulate matters (PM) in exhaust gas discharged from an engine, several exhaust gas purification apparatus have been proposed. In these exhaust gas purification apparatus, a reduction catalyst is placed in the exhaust system of the engine, and a reducing agent is injection-supplied into an exhaust gas passage on the upstream side of the reduction catalyst, to thereby catalytically reduction react the NOx in the exhaust gas with the reducing agent, and thus purification process is applied to convert the NOx into harmless constituents. The reducing agent is stored in a liquid state at room temperature in a reservoir tank, and a necessary amount thereof is injection-supplied from an injection nozzle. The reduction reaction uses ammonia which has excellent reactivity with NOx, and an aqueous solution of reducing agent such as a urea aqueous solution, ammonia aqueous solution or the like that is hydrolyzed to produce ammonia easily is used as the reducing agent (refer for example to Japanese Unexamined Patent Publication No. 2000-27627 and Japanese Unexamined Patent Publication No. 2001-173431).
In such an exhaust gas purification apparatus, as shown in FIG. 6, an injection nozzle 2 is arranged substantially parallel with a direction A of the exhaust gas flow inside an exhaust pipe 1 of an exhaust system so as to face the downstream side, and a reducing agent such as a urea aqueous solution is ejected from the injection nozzle 2 substantially orthogonal with respect to the exhaust gas flow direction A as indicated by arrows B and C. Moreover, as shown in FIG. 7, in the vicinity of an end portion of the injection nozzle 2 on the exhaust gas downstream side, injection holes 3 are drilled outward from the central axis and substantially orthogonal thereto, inside a thick portion of a nozzle main body.
However in the aforementioned conventional exhaust gas purification apparatus, since as shown in FIG. 7, the injection holes 3 of the injection nozzle 2 are drilled inside the thick portion of the nozzle main body, and the injection holes 3 open directly to the outer peripheral surface of the nozzle main body, when injection of the urea aqueous solution from the injection nozzle 2 stops, then as shown in FIG. 8A, in some cases a urea aqueous solution 4 becomes attached to the injection holes 3 and remains in the vicinity thereof, on the outer peripheral surface of the nozzle main body. Moreover, as shown in FIG. 8B, since chamfers 5 tilted inward are formed on the inner peripheral edge where the injection holes 3 open on the outer peripheral surface of the nozzle main body, it is possible that the urea aqueous solution 4 remaining on the outer peripheral surface of the nozzle main body enters into the injection holes 3 along the inclination of the chamfers 5, and dries out and solidifies inside the injection holes 3. In such cases, the injection holes 3 of the nozzle 2 become clogged so that sufficient reducing agent cannot be supplied into the exhaust gas passage on the upstream side of the reduction catalyst, which results in a decrease in the efficiency of purification processing of the NOx into harmless constituents.
Furthermore, since as shown in FIG. 7, the injection holes 3 are provided by drilling, so that these holes 3 are directed outward from the central axis of the nozzle main body and substantially orthogonal thereto, and so that the urea aqueous solution is ejected from the injection nozzle 2 in the directions indicated by the arrows B and C (see FIG. 6) substantially orthogonal to the exhaust gas flow direction A. As a result, the urea aqueous solution is unlikely to follow the exhaust gas flow. Particularly where the exhaust gas amount is small, it is possible that the urea aqueous solution 4 ejected from the injection holes 3 in FIG. 6, becomes attached to the internal surface of the exhaust pipe 1 and urea is deposited, and the urea aqueous solution 4 decreases by the amount of urea deposited, so that the mixing proportion of the urea aqueous solution 4 and the exhaust gas decreases, resulting in a decrease in the efficiency of the NOx purification processing in some cases.